The goal of this program project is to investigate the manner in which coordinated movement results from interactions between the musculoskeletal system and circuits in the spinal cord. The purpose of Project IV is to address two aspects of this interaction. First, data from the previous grant cycle suggests that rapid sensory feedback from muscle receptor is required from coordinated movement. This requirement is greatest during phases of movement that require muscles to provide braking action and active lengthening occurs during walking down a ramp. We will obtain further support fr this hypothesis and determine the sources of the required feedback for extensor muscles of the ankle in the cat. Second, we will explore the unique mechanical actions of the ankle extensors in both humans and cats in the performance of turning movements, and the role of sensory feedback in coordinating the activation of these muscles. This project addresses the extent to which the nervous system exploits seemingly subtle differences in mechanical action to achieve different behaviors and how sensory information contributes to the resulting coordination. In order to study the contributions of sensory pathways from muscles with specific mechanical actions, the nerves supplying these muscles will be transected and re- sutured. After re-growth of the motor supply to the muscle, the sensory feedback has been found to be markedly impaired. Motor deficits in animals with this lesion will be investigated during locomotion of varying aspects of level surfaces and walking up and down ramps. The ability of these animals to compensate for the sensory deficit will be explored. Contributions to turning by muscles crossing the ankle will be investigated in human subjects and cats using fine wire electromyography, ground reaction force measurements, and 3 dimensional kinematics. The results of this project will enhance understanding of the physiological role of proprioceptive feedback and motor deficits that result from peripheral nerve injury. The results will also impact therapies that involve the manipulation of destruction of sensory feedback and tendon transfer procedures.